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From "stack (JIRA)" <j...@apache.org>
Subject [jira] Commented: (HBASE-1148) Always flush HLog on root or meta region updates
Date Fri, 23 Jan 2009 19:41:59 GMT

    [ https://issues.apache.org/jira/browse/HBASE-1148?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12666668#action_12666668
] 

stack commented on HBASE-1148:
------------------------------

I was thinking on this last night and I suppose its not going to be easy.  An HRS carries
the HLog.  We'd have to make it so IF THE EDIT IS FOR THE CATALOG TABLE, ignore general flush
settings and write out the .META. and -ROOT- edits.

I wonder what effect this will have on performance?  If it slows things down, thats probably
ok since catalog edits are relatively rare but I wonder if it would bring the server hosting
catalog regions to a crawl?

> Always flush HLog on root or meta region updates
> ------------------------------------------------
>
>                 Key: HBASE-1148
>                 URL: https://issues.apache.org/jira/browse/HBASE-1148
>             Project: Hadoop HBase
>          Issue Type: Bug
>          Components: regionserver
>    Affects Versions: 0.19.0, 0.19.1, 0.20.0
>            Reporter: Jim Kellerman
>            Assignee: Jim Kellerman
>            Priority: Critical
>
> Flushing an HLog does not currently guarantee that the updates will be visible (see HADOOP-4379),
however in the case of root or meta region updates, this is critical.
> I was able to create a situation by killing both the root and meta region servers, from
which the cluster recovered, but because of the missed edits, clients found the old parent
region rather than the new child regions because the fact that the parent region had split
was not in the HLog of the crashed region servers (the master knew because of the MSG_REGION_SPLIT
message it received) but the clients read the meta table and because that change was lost,
clients were trying to find the parent region.
> So, when a SequenceFile.Writer.sync() guarantees that what has been written will be visible
to new readers, we need to modify HLog so that if it is writing an update to the root or meta
regions, that it immediately flushes (syncs) the log file so that the changes will be visible
when the log file is recovered.

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